This invention relates generally to sugar centrifugals and more particularly to devices and methods for achieving improvements in continuous sugar centrifugals which provide reduced lump formation together with a virtual elimination of crystal impact damage.
Massecuite is separated into sugar crystals and molasses (or run-off), its two components, through several operations. In cases where high-grade sugar crystals are being separated, the quality and integrity of the crystals after separation is of great importance. Broken crystals require downgrading of sugar product, and they must, therefore, be avoided. However, the viscosity of the molasses component of the massecuite requires centrifugal separation in order to free the sugar crystals from the surrounding syrup. The high discharge velocity and resultant high gravity forces imposed on the massecuite by the centrifugals frequently results in excessive damage to the high grade crystals due to high velocity impact with the bare wall of the sugar housing and with other crystals which reach the wall earlier. This crystal damage promotes lump formation, may require shut-down of the centrifugal for removal of lump build-up. Another contributor to lump formation is a build-up of a syrup film on the surface on which the crystals impact when discharged from the centrifuge basket. The film results from the extremely small fraction of the molasses which remains on the sugar crystals after centrifuging. Especially in the case of continuous centrifugals, over a long operating time, the syrup film can become quite thick and sticky. This makes lump formation increasingly likely, thereby limiting the economic benefit of continuous operation.
Since crystal damage and lump formation are more pronounced in continuous centrifugals than they are in batch type centrifugals beyond some threshold running time, continuous operation does not achieve its full potential of improved production efficiency for high grade sugar. Therefore, it is very important to operate continuous centrifugals in such a manner and to provide such features as will minimize, if not eliminate, crystal damage and lump formation to improve performance in continuous centrifugals.
Efforts to avoid such damage and the attendant economic losses have included reduction of basket rotary speed, which reduces crystal/molasses separation efficiency, and provision of a deflecting shock absorber designed to cushion crystal impact with the sugar housing and to deflect the crystals out of the path of trailing crystals to prevent crystal-to-crystal impact damage and lump formation and build up. The deflector is also capable of air-induced vibration to prevent sticking by the impinging crystals. The deflector/vibrator is more fully disclosed in U.S. patent application Ser. No. 07/808,415, filed Dec. 13, 1991 and commonly assigned by the inventor herein. These efforts have improved performance of sugar centrifugals, but they have not fully eliminated crystal breakage and lump formation. Particularly in continuous centrifugals, some lump formation and breakage persists. Because of continuous operation, a significant deposit of syrup may form, over time, on the surface of the deflector/shock absorber. This may retard sloughing of the first-arriving crystals so that they are struck by following crystals and mutually damaged. Lump formation begins with the earliest crystal damage and continues at an increasing rate during operation. The aggravating effects of continuous operation are therefore expected.
Reduction of speed of the centrifugal results in a reduction of throughput without completely eliminating crystal breakage even when crystal separation is assisted by differential air pressure through the centrifugal basket as disclosed in U.S. patent application Ser. No. 07/806,521, filed Dec. 13, 1991 and commonly assigned by the inventor herein. Although improved performance is achieved using differential air pressure, the ultimate aim of lump-free undamaged crystals is still not realized.
Another approach to minimizing crystal damage was disclosed in U.S. patent application Ser. No. 07/807,284, filed Dec. 13, 1991 and also commonly assigned by the inventor herein. In this case, the effective centrifugal speed was reduced by providing a crystal release ring having a diameter significantly less than that of the upper shroud ring of the centrifugal basket. This reduced the tangential velocity of the crystals at their point of release from the basket and, thereby reduced impact energy and resultant damage to the crystals. Again, although crystal damage and lump formation were reduced, they still occurred to an undesirable extent for high grade sugar production.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.